Had a problem with a WHM/cPanel server where it was working fine from a couple different networks, but then would not work on others. The server itself seemed fine and fully operational.

Checked firewall rules on routers, server, checked IP routes, tried disabling cPHulk. Ended up being there were a couple addresses added with the incorrect subnet mask which was keeping it from working. Removed the IP’s with the wrong subnet and it started working on all networks.

Check and verify that DNS is not being blocked upstream by a firewall. Behavior is weird, the server can get out to the internet, access to the servers IP address is available, but can’t ping domains that are locally hosted. Are also unable to ping the domains from the internet in.

To resolve the issue either disable the DNS firewall rules, or better yet add some rules to allow access to the cPanel server.